1. Field of the Invention
This invention relates to a magnetometer and in particular to a Micro-electro mechanical systems (MEMS) self-resonant magnetometer.
2. Discussion of Prior Art
Resonant magnetometers are well known. One of the earliest MEMS based resonant magnetometers is described by D. K. Wickenden et al in the paper “MEMS based resonating Xylophone Bar Magnetometers”, SPIE Conference Proceedings—Micromachined Devices and Components IV, SPIE Vol. 3514, pp 350-358, 1998. The Wickenden et al device comprises a surface micro-machined bar that is fixed to a pair of electrodes at the nodes of its first resonant mode. In use, an alternating current (AC) is passed through the bar at the resonant frequency of the bar. In the presence of a magnetic field, the Lorenz force causes the bar to resonate and the magnitude of any such movement is sensed capacitively to provide an indication of the strength of the applied magnetic field.
A variation to the basic MEMS resonant magnetometer design has recently been described by Zaki Izham, Michael C L Ward, Kevin M Brunson and Paul C Stevens in the paper entitled “Development of a Resonant Magnetometer”; see Proceedings of the 2003 Nanotechnology Conference and Trade show, February 23-27, San Francisco; Volume 1, pp 340-343, ISBN 0-9728422-0-9. The Izham et al resonant magnetometer is formed from a silicon-on-insulator (SOI) wafer and comprises a oscillatory mass having two sets of fixed-fixed suspensions that allow it to move along an axis in the plane of the wafer. An AC current having a frequency around the resonant frequency of the oscillatory mass is passed along the suspension thereby causing the mass to resonate in the presence of a magnetic field. A set of electrodes are attached to the mass to allow the magnitude of any magnetic field induced movement to be measured capacitively.
To maximise Q amplification in a resonant magnetometer, it is necessary to ensure that the frequency of the AC current supplied to the oscillatory mass matches, or is sufficiently close to, the resonant frequency thereof. Although the resonant frequency of a beam can be predicted theoretically and/or measured, variations may arise from temperature variations, stress induced effects and/or any non-inearities that are present in the resonant beam suspension. If an AC current is applied that has a frequency away from the resonant condition, the sensitivity of the device will be greatly diminished due to the loss of the Q amplification of the Lorentz force.
To ensure the resonant beam is driven into resonance, it is known to adjust the output frequency of the frequency generator in response to the output of a pick-up circuit that is used to sense the oscillation frequency of the beam. A phase-lock loop is then used to ensure that the frequency of the applied AC tracks any change in the oscillation frequency of the resonant beam. However, phase-lock loop circuits can introduce unwelcome phase noise because they must constantly search for the optimum frequency.
In high sensitivity applications, such as compasses and the like, the resonant magnetometer requires a mechanical Quality factor (Q) of between about 500 to 5000 and a resonant frequency within the range of around 500 Hz to 30 kHz. This high Q factor means that the frequency generator used to supply the AC current to the resonant beam structure needs to have an accuracy better than 1 Hz in several kHz. The provision of such a high resolution frequency generator, in addition to the provision of phase lock loop circuitry, adds cost and complexity to the control electronics that are required to operate the device.